1. Field of the Invention
The present invention relates to a light source that can output a broad-band lightwave.
2. Description of the Background Art
Light sources that can output a broad-band lightwave are useful in a field of optical measurement, for example. It is known that such light sources include a light source that outputs a lightwave generated by the amplified spontaneous emission (hereinafter referred to as an ASE lightwave). For example, the published Japanese patent application Tokukai 2002-329907 has disclosed a light source that uses an erbium-doped fiber (EDF) as an optical active element. FIG. 11 is a schematic diagram showing a conventional broad-band light source 8 disclosed in Tokukai 2002-329907. The light source 8 comprises (a) an optical waveguide including an EDF 81, (b) an optical coupler 82, an optical isolator 83, and an optical connector 88, all of which are placed at a first-end side of the optical waveguide, and (c) a reflector 89 placed at a second-end side of the optical waveguide.
In the broad-band light source 8, a pumping lightwave outputted from a pumping light source 84 enters the EDF 81 via the optical coupler 82 to generate an ASE lightwave in the EDF 81. In the EDF 81's region in the vicinity of the optical coupler 82, the population inversion is as relatively high as about 60% or more, for example. As a result, a gain peak lies in the C-band (1,530 to 1,565 nm). On the other hand, in the region far from the optical coupler 82, the population inversion is as relatively low as about 40%, for example. As a result, a gain peak lies in the L-band (1,565 to 1,625 nm). The ASE lightwave generated in the EDF 81 enters the optical isolator 83 either directly or after being reflected at the reflector 89. After passing through the optical isolator 83, the ASE lightwave is outputted from the optical connector 88. In other words, the light source 8 can output from the optical connector 88 an ASE lightwave lying over a broad band including both the C- and L-bands.
However, in the broad-band light source 8, the reflector 89 and the optical isolator 83 sandwiching the EDF 81 constitute a resonator, raising the possibility of oscillation. To prevent the oscillation, it is possible to design a broad-band light source 9 having a structure shown in FIG. 12. FIG. 12 is a schematic diagram showing a broad-band light source 9. The light source 9 comprises (a) a first optical waveguide including an EDF 91a, (b) an optical coupler 92a and an optical isolator 93a, both of which are placed at a first-end side of the first optical waveguide, (c) a second optical waveguide including an EDF 91b, (d) an optical coupler 92b and an optical isolator 93b, both of which are placed at a first-end side of the second optical waveguide. Second ends of the first and second optical waveguides are open ends.
A pumping lightwave outputted from a pumping light source 94a enters the EDF 91a. An adjustment is performed such that the level of population inversion in the EDF 91a becomes relatively high, so that an ASE lightwave having an intensity peak in the C-band is generated in the EDF 91a. A pumping lightwave outputted from a pumping light source 94b enters the EDF 91b. An adjustment is performed such that the level of population inversion in the EDF 91b becomes relatively low, so that an ASE lightwave having an intensity peak in the L-band is generated in the EDF 91b. The ASE lightwaves generated in the EDFs 91a and 91b are combined by an optical coupler 96. The resultant ASE lightwave is outputted from an optical connector 98. In other words, the light source 9 can output from the optical connector 98 an ASE lightwave lying over a broad band including both the C- and L-bands.
As described above, the broad-band light source 8 may oscillate. On the other hand, the broad-band light source 9 reduces the possibility of oscillation, because the second ends of the first and second optical waveguides are open ends. However, its intensity ratio of the output lightwave to the pumping lightwave is low, that is, it has low efficiency.